This invention relates to radiation detectors employing rare-earth-doped ceramic scintillators. More particularly, it relates to scintillator materials having improved afterglow characteristics.
Radiation detectors have long been used to detect penetrating radiation in such applications as x-ray counters and image intensifiers. More recently, such detectors have played an important role in computerized tomography (CT) scanners, digital radiography (DR), and other x-ray, gamma radiation, ultraviolet radiation, and nuclear radiation detecting applications.
Computerized tomography scanners are medical diagnostic instruments in which a subject is exposed to a relatively planar beam or beams of x-ray radiation, with the intensity of the beam varying in direct relationship to the energy absorption along a plurality of subject body paths. By measuring the x-ray intensity (i.e., the x-ray absorption) along these paths from a plurality of different angles or views, x-ray absorption coefficients can be computed for various areas in any plane of the body through which the radiation passes. These areas typically comprise approximately a square portion of about 1 mm. by 1 mm. The absorption coefficients are used to produce a display of the object being intersected by the x-ray beam, such as, for example, the bodily organs of a human subject.
An integral and important part of the scanner is the detector which receives the x-ray radiation that has been modulated by passage through the particular body under study. The x-ray detector generally contains a scintillator material which, when excited by the impinging x-ray radiation, emits optical wavelength energy. In typical medical or industrial applications, the optical output from the scintillator material is made to impinge upon a photoelectrically responsive material which produces electrical output signals. The amplitude of these signals is directly related to the intensity of the impinging x-ray radiation. The electrical signals are digitized for processing by digital computer means, which means generates the absorption coefficients in a form suitable for display on a cathode ray tube screen or on other permanent media.
In order to meet the specific and demanding requirements of computerized tomography applications, the scintillator material employed must be an efficient converter of x-ray radiation into optical radiation, in those regions of the electromagnetic spectrum which are most efficiently detected by photosensors such as photomultipliers or photodiodes. It is also desirable that the scintillator transmit the optical radiation efficiently, by avoiding optical trapping, so that optical radiation originating deep inside the scintillator body escapes for detection by externally situated photodetectors. The scintillator material should also have high x-ray stopping power, low hysteresis, spectral linearity, and short afterglow or persistence. High x-ray stopping power is desirable for efficient x-ray detection, because x-rays not absorbed by the scintillator escape detection. Hysteresis refers to the scintillator material property whereby the optical output varies for identical x-ray excitation, based on the irradiation history of the scintillator. For CT applications, typical detecting accuracies are on the order of one part in one thousand, for a number of successive measurements taken at a relatively high rate. Accordingly, low hysteresis is required in order to provide repeated precise measurements of optical output from each scintillator cell, and to provide substantially identical optical outputs for identical x-ray radiation exposure impinging on the scintillator body. Spectral linearity is important because x-rays impinging on the scintillator body typically include a number of different frequencies, and because the scintillator response to the radiation should be substantially uniform for all such frequencies. Afterglow or persistence is the tendency of the scintillator to continue emitting optical radiation for a time after termination of the x-ray excitation. Long afterglow results in blurring, with time, of the information-bearing signal. Furthermore, for applications requiring rapid sequential scanning, such as, for example, in imaging moving bodily organs, short afterglow is essential for rapid cycling of the detector.
Polycrystalline ceramic scintillators which exhibit many of the desirable properties outlined above are described in U.S. Pat. No. 4,421,671, issued Dec. 20, 1983 to D. Cusano et al, and in U.S. application Ser. No. 629,027, filed Jul. 9, 1984 in the name of D. Cusano et al and assigned to the present assignee (continuation of U.S. application Ser. No. 389,812 filed Jun. 18, 1982). The scintillators described therein are comprised of yttria and gadolinia, and include at least one of a variety of rare earth activators for enhancing luminescent efficiency. The scintillator composition may also include one or more of several disclosed transparency promoters and light output restorers. The above-referenced patents also disclose that luminescent afterglow of the yttria-gadolinia ceramic scintillators described therein may be reduced by adding ytterbium oxide (Yb.sub.2 O.sub.3), strontium oxide (SrO), or calcium oxide (CaO). However, as CT systems are improved, meeting the demand for faster response times requires scintillator materials having improved afterglow.
Accordingly, it is an object of the present invention to provide a radiation detector exhibiting improved afterglow characteristics when used with a source of penetrating radiation.
It is another object of the present invention to provide a rare-earth-doped polycrystalline ceramic scintillator which is especially useful for CT, DR, and other x-ray detecting applications.
It is a further object of the present invention to provide a polycrystalline yttria-gadolinia ceramic scintillator exhibiting reduced luminescent afterglow.
It is also an object of the present invention to provide methods for preparing such scintillator bodies.